Many patients suffer from the occurrence of a heart block, in which an electrical signal propagating from the sinoatrial node is stopped at the atrioventricular valve or just below the atrioventricular node. The heart block effectively blocks the electrical signal from reaching the ventricles and causing them to contract. In some situations, the heart block may not fully obstruct the electrical signal from propagating to the ventricles. In these instances, conduction may be slowed, or may be of intermittent nature. Implantable medical devices (IMDs), such as pacemakers, may detect the intrinsic ventricular activity via electrodes placed within or around the heart.
When the heart block succeeds in fully blocking the electrical signal from reaching the ventricles, a pacemaker detects the absence of intrinsic electrical activity in the heart and applies a pacing pulse to stimulate the ventricles to contract. Pacemakers use various algorithms to determine when to apply a pacing pulse. Ideally, a pacemaker should only apply a pacing pulse when intrinsic ventricular activity does not occur within the heart.
This strategy is ideal because hemodynamic performance and the battery life of the pacemaker are increased when intrinsic ventricular activity occurs as opposed to the delivery of a pacing by the pacemaker delivering a pacing pulse to stimulate the ventricles. Thus, it is beneficial to detect the presence of intrinsic ventricular activity.
Intrinsic ventricular activity following a pace is understood to be either fusion or pseudo fusion. In the case of pseudo-fusion, the pacemaker fails to detect a depolarization and a stimulus is delivered during or after the intrinsic depolarization, but it has no capture because the cardiac tissue is already refractory, there is a ventricular sense, or a pace was given while the heart was contracting due to a natural trigger.
Pacemakers may determine occurrence of intrinsic ventricular activity by extending a pacing interval between a delivered pacing pulse and a subsequently delivered pacing pulse. This process is sometimes referred to as hysteresis. The pacemaker generally schedules the extended pacing interval to occur once per time-unit or once per number of cycles. During the scheduled extended pacing interval, the pacemaker monitors the heart for intrinsic ventricular activity. In the event that intrinsic ventricular activity occurs is detected, the pacemaker does not apply a pacing to the ventricle of the heart. Otherwise, at the end of the extended pacing interval, the pacemaker delivers a pacing pulse to stimulate contraction of the ventricle.
One of the advances provided by utilizing the present invention rather than a common scan method to detect intrinsic activity is that no symptomatic large interval extension is needed, while the chance of detecting intrinsic activity is significantly larger than when using a (non-symptomatic) small interval extension.